Austenitic Heat Resistant Alloy and Method for Producing Same

ABSTRACT

Provided is an austenitic heat resistant alloy having a chemical composition consisting of, in mass %: C: 0.02 to 0.12%; Si: 2.0% or less; Mn: 3.0% or less; P: 0.030% or less; S: 0.015% or less; Cr: 20.0% or more and less than 28.0%; Ni: more than 35.0% and 55.0% or less; Co: 0 to 20.0%; W: 4.0 to 10.0%; Ti: 0.01 to 0.50%; Nb: 0.01 to 1.0%; Mo: less than 0.50%; Cu: less than 0.50%; Al: 0.30% or less; N: less than 0.10%; Mg: 0 to 0.05%; Ca: 0 to 0.05%; REM: 0 to 0.50%; V: 0 to 1.5%; B: 0 to 0.01%; Zr: 0 to 0.10%; Hf: 0 to 1.0%; Ta: 0 to 8.0%; Re: 0 to 8.0%; and the balance: Fe and impurities, wherein a shortest distance from a center portion to an outer surface portion of a cross section of the alloy is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy, an austenite grain size number at the outer surface portion is −2.0 to 4.0, an amount of Cr which is present as a precipitate satisfies [CrPB/CrPS≤10.0], and [YSS/YSB≤1.5] and [TSS/TSB≤1.2] are satisfied at a normal temperature.

TECHNICAL FIELD

The present invention relates to an austenitic heat resistant alloy anda method for producing the same.

Conventionally, for thermal power generation boilers, chemical plantsand the like which are used in a high temperature environment, 18-8austenitic stainless steels, such as SUS304H, SUS316H, SUS321H, andSUS347H, have been used as materials for apparatuses.

In recent years, however, ultra super critical boilers, wheretemperature and pressure of steam are increased to enhance efficiency,have been newly installed worldwide. The use conditions of apparatusesin such a high temperature environment have become extremely severe, andtherefore, properties which materials being used are required to possesshave become strict. Under such circumstances, using 18-8 austeniticstainless steel, which is conventionally used, has become extremelyinsufficient in terms of not only corrosion resistance but also hightemperature strength, particularly creep rupture strength.

To overcome the above problems, various studies have been made. Forexample, Patent Documents 1 to 4 disclose austenitic steel excellent inhigh temperature strength and corrosion resistance. Further, PatentDocument 5 discloses austenitic stainless steel excellent in hightemperature strength and corrosion resistance. According to PatentDocuments 1 to 5, the amount of Cr is increased to 20% or more, and Wand/or Mo are contained so as to enhance high temperature strength.

LIST OF PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP61-179833A

Patent Document 2: JP61-179834A

Patent Document 3: JP61-179835A

Patent Document 4: JP61-179836A

Patent Document 5: JP2004-3000A

SUMMARY OF INVENTION Technical Problem

Large-sized structural members made of a material for apparatuses, suchas thermal power generation boilers or chemical plants, are hot rolledor hot forged and then subjected to final heat treatment without coldrolling before putting into use. Accordingly, the grain size isrelatively large. For this reason, usually, there is a problem that 0.2%proof stress and tensile strength at a normal temperature, which aredefined as the specifications of materials, are lower than those of amaterial obtained by performing final heat treatment after cold rolling.

In addition to the above, in a large-sized structural member, a coolingspeed at the time of performing heat treatment varies largely fromregion to region and hence, there is a variation from region to regionin the amount of solid solution elements which contribute tostrengthening the member as precipitates during use at a hightemperature. There is also a problem that creep rupture strength variesdue to such variation. Accordingly, it is difficult to adopt steeldisclosed in Patent Documents 1 to 5 to a large-sized structural member.

The present invention has been made to overcome the above problems, andan objective of the present invention is to provide an austenitic heatresistant alloy and a method for producing the same which exhibitssufficient 0.2% proof stress and tensile strength at a normaltemperature, and sufficient creep rupture strength at a high temperaturein large-sized structural members.

Solution to Problem

The present invention has been made to overcome the above problems, andthe gist of the present invention is the following austenitic heatresistant alloy and method for producing the same.

(1) An austenitic heat resistant alloy having a chemical compositionconsisting of, in mass %:

C: 0.02 to 0.12%;

Si: 2.0% or less;

Mn: 3.0% or less;

P: 0.030% or less;

S: 0.015% or less;

Cr: 20.0% or more and less than 28.0%;

Ni: more than 35.0% and 55.0% or less;

Co: 0 to 20.0%;

W: 4.0 to 10.0%;

Ti: 0.01 to 0.50%;

Nb: 0.01 to 1.0%;

Mo: less than 0.50%;

Cu: less than 0.50%;

Al: 0.30% or less;

N: less than 0.10%;

Mg: 0 to 0.05%;

Ca: 0 to 0.05%;

REM: 0 to 0.50%;

V: 0 to 1.5%;

B: 0 to 0.01%;

Zr: 0 to 0.10%;

Hf: 0 to 1.0%;

Ta: 0 to 8.0%;

Re: 0 to 8.0%; and

the balance: Fe and impurities, wherein

a shortest distance from a center portion to an outer surface portion ofa cross section of the alloy is 40 mm or more, the cross section beingperpendicular to a longitudinal direction of the alloy,

an austenite grain size number at the outer surface portion is −2.0 to4.0,

an amount of Cr which is present as a precipitate obtained by anextraction residue analysis satisfies a following formula (i), and

mechanical properties at a normal temperature satisfy following formula(ii) and formula (iii):

Cr_(PB)/Cr_(PS)≤10.0   (i)

YS_(S)/YS_(B)≤1.5   (ii)

TS_(S)/TS_(B)≤1.2   (iii)

where meaning of each symbol in the formulas is as follows:

Cr_(PB): amount of Cr which is present at center portion as precipitateobtained by extraction residue analysis

Cr_(PS): amount of Cr which is present at outer surface portion asprecipitate obtained by extraction residue analysis

YS_(B): 0.2% proof stress at center portion

YS_(S): 0.2% proof stress at outer surface portion

TS_(B): tensile strength at center portion

TS_(S): tensile strength at outer surface portion.

(2) The austenitic heat resistant alloy described in the above (1),wherein the chemical composition contains one or more elements selectedfrom a group consisting of, in mass %:

Mg: 0.0005 to 0.05%;

Ca: 0.0005 to 0.05%;

REM: 0.0005 to 0.50%;

V: 0.02 to 1.5%;

B: 0.0005 to 0.01%;

Zr: 0.005 to 0.10%;

Hf: 0.005 to 1.0%;

Ta: 0.01 to 8.0%; and

Re: 0.01 to 8.0%.

(3) The austenitic heat resistant alloy described in the above (1) or(2), wherein 10,000-hour creep rupture strength at 700° C. in thelongitudinal direction at the center portion is 100 MPa or more.

(4) A method for producing an austenitic heat resistant alloy, themethod including the steps of:

performing hot working on an ingot or a cast piece having the chemicalcomposition described in the above (1) or (2); and

thereafter performing heat treatment where the ingot or the cast pieceis heated to a heat-treatment temperature T (° C.) ranging from 1100 to1250° C., is held for 1000 D/T to 1400 D/T (min), and is cooled withwater,

wherein symbol “D” denotes a maximum value (mm) of a linear distancebetween an arbitrary point on an outer edge of a cross section of thealloy and another arbitrary point on the outer edge, the cross sectionbeing perpendicular to a longitudinal direction of the alloy.

(5) The method for producing an austenitic heat resistant alloydescribed in the above (4), wherein

in the step of performing the hot working, the working is performed oneor more times in a direction substantially perpendicular to thelongitudinal direction.

Advantageous Effects of Invention

The austenitic heat resistant alloy of the present invention has smallvariation in mechanical properties from region to region, and isexcellent in creep rupture strength at a high temperature.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the respective requirements of the present invention aredescribed in detail.

1. Chemical Composition

The reasons for limiting respective elements are as follows. In thedescription made hereinafter, symbol “%” for content refers to “mass %”.

C: 0.02 to 0.12%

C (carbon) forms carbides so that C is an indispensable element formaintaining high temperature tensile strength and creep rupture strengthrequired for an austenitic heat resistant alloy. Accordingly, it isnecessary to set a content of C to 0.02% or more. However, when the Ccontent exceeds 0.12%, not only undissolved carbides are formed, butalso Cr carbides increase and hence, mechanical properties, such asductility and toughness, and weldability deteriorate. Accordingly, the Ccontent is set to a value ranging from 0.02 to 0.12%. The C content ispreferably 0.05% or more and 0.10% or less.

Si: 2.0% or less

Si (silicon) is contained as a deoxidizing element. Further, Si is anelement effective in increasing oxidation resistance, steam oxidationresistance and the like. Si is also an element which facilitates theflow of a casting material. However, when a content of Si exceeds 2.0%,the formation of intermetallic compounds, such as a σ phase, is promotedand hence, stability of micro-structure at a high temperaturedeteriorates, thus lowering toughness and ductility. When the Si contentexceeds 2.0%, weldability is also lowered. Accordingly, the Si contentis set to 2.0% or less. When importance is placed on structuralstability, the Si content is preferably set to 1.0% or less. When adeoxidizing action is sufficiently ensured by other elements, it is notparticularly necessary to set the lower limit of the Si content.However, when importance is placed on a deoxidizing action, oxidationresistance, steam oxidation resistance and the like, the Si content ispreferably set to 0.05% or more, and more preferably set to 0.10% ormore.

Mn: 3.0% or less

Mn (manganese) has a deoxidizing action in the same manner as Si, andalso has an action of fixing S, which is inevitably contained in thealloy, as a sulfide, thus improving ductility at a high temperature.However, when a content of Mn exceeds 3.0%, the precipitation ofintermetallic compounds, such as a σ phase, is promoted and hence,structural stability, and mechanical properties, such as hightemperature strength, deteriorate. Accordingly, the Mn content is set to3.0% or less. The Mn content is preferably 2.0% or less, and morepreferably 1.5% or less. It is not necessary to set the lower limit ofthe Mn content. However, when importance is placed on an action ofimproving ductility at a high temperature, the Mn content is preferablyset to 0.10% or more, and more preferably set to 0.20% or more.

P: 0.030% or less

P (phosphorus) is inevitably mixed in the alloy as an impurity, andremarkably lowers weldability and ductility at a high temperature.Accordingly, a content of P is set to 0.030% or less. It is preferableto reduce the P content to as much as possible. The P content ispreferably set to 0.020% or less, and more preferably set to 0.015% orless.

S: 0.015% or less

S (sulfur) is inevitably mixed in the alloy as an impurity in the samemanner as P, and remarkably lowers weldability and ductility at a hightemperature. Accordingly, a content of S is set to 0.015% or less. Whenimportance is placed on hot workability, the S content is preferably setto 0.010% or less, more preferably set to 0.005% or less, and furtherpreferably set to 0.003% or less.

Cr: 20.0% or more and less than 28.0%

Cr (chromium) is an important element which excellently exhibits anaction of improving corrosion resistance, such as oxidation resistance,steam oxidation resistance, and high temperature corrosion resistance.However, when a content of Cr is less than 20.0%, these advantageouseffects cannot be obtained. On the other hand, when the Cr contentincreases, particularly to 28.0% or more, the micro-structure is madeunstable due to the precipitation of a σ phase or the like, andweldability also deteriorates. Accordingly, the Cr content is set to avalue ranging of 20.0% or more and less than 28.0%. The Cr content ispreferably 21.0% or more, and more preferably 22.0% or more. Further,the Cr content is preferably 26.0% or less, and more preferably 25.0% orless.

Ni: more than 35.0% and 55.0% or less

Ni (nickel) is an element which makes the austenitic structure stable,and is also an element important to ensure corrosion resistance. Tomaintain the balance with the Cr content, it is necessary to set acontent of Ni to more than 35.0%. On the other hand, excessively high Nicontent increases costs and hence, the Ni content is set to 55.0% orless. The Ni content is preferably 40.0% or more, and more preferably42.0% or more. Further, the Ni content is preferably 50.0% or less, andmore preferably 48.0% or less.

Co: 0 to 20.0%

It is not always necessary to contain Co (cobalt). However, in the samemanner as Ni, Co makes the austenitic structure stable, and alsocontributes to enhancing creep rupture strength. Accordingly, Co may becontained in lieu of a part of Ni. However, when a content of Co exceeds20.0%, the effect is saturated and hence, economic efficiency islowered. Accordingly, the Co content is set to a value ranging from 0 to20.0%. The Co content is preferably 15.0% or less. When it is desired toobtain the advantageous effects, the Co content is preferably set to0.5% or more.

W: 4.0 to 10.0%

W (tungsten) is dissolved in a matrix, thus not only contributing toenhancing creep rupture strength as a solid-solution strengtheningelement, but also precipitating as a Fe₂W Laves phase or a Fe₇W₆ μ phaseso that creep rupture strength is significantly enhanced. Accordingly, Wis an important element. However, when a content of W is less than 4.0%,the advantageous effects cannot be obtained. On the other hand, even ifthe W content is set to more than 10.0%, an effect of enhancing strengthis saturated, and structural stability and ductility at a hightemperature deteriorate. Accordingly, the W content is set to a valueranging from 4.0 to 10.0%. The W content is preferably 5.0% or more, andmore preferably 5.5% or more. Further, the W content is preferably 9.0%or less, and more preferably 8.5% or less.

Ti: 0.01 to 0.50%

Ti (titanium) is an element which forms carbo-nitrides, thus having aneffect of enhancing creep rupture strength. However, when a content ofTi is less than 0.01%, sufficient effects cannot be obtained. On theother hand, when the Ti content exceeds 0.50%, ductility at a hightemperature is lowered. Accordingly, the Ti content is set to a valueranging from 0.01 to 0.50%. The Ti content is preferably set to 0.05% ormore, and more preferably set to 0.10% or more. Further, the Ti contentis preferably set to 0.40% or less, and more preferably set to 0.35% orless.

Nb: 0.01 to 1.0%

Nb (niobium) has an action of forming carbo-nitrides, thus enhancingcreep rupture strength. However, when a content of Nb is less than0.01%, sufficient effects cannot be obtained. On the other hand, whenthe Nb content exceeds 1.0%, ductility at a high temperature is lowered.Accordingly, the Nb content is set to a value ranging from 0.01 to 1.0%.The Nb content is preferably 0.10% or more. Further, the Nb content ispreferably 0.90% or less, and more preferably 0.70% or less.

Mo: less than 0.50%

Mo (molybdenum) is an element which is dissolved in a matrix, thuscontributing to enhancing creep rupture strength as a solid-solutionstrengthening element and hence, Mo has been conventionally consideredas an element having substantially the same action as W. However, theinventors of the present invention have made studies, and found thefollowing. When Mo is contained in combination in an alloy whichcontains the amounts of W and Cr, a σ phase may precipitate afterlong-term use and hence, creep rupture strength, ductility and toughnessmay be lowered. Accordingly, it is desirable to reduce a content of Moas much as possible, and the Mo content is set to less than 0.50%. It ispreferable to limit the Mo content to less than 0.20%.

Cu: less than 0.50%

In the present invention, Cu (copper) lowers a fusing point, thuslowering hot workability and weldability. Accordingly, it is desirableto reduce a content of Cu as much as possible, and the Cu content is setto less than 0.50%. It is preferable to limit the Cu content to lessthan 0.20%.

Al: 0.30% or less

Al (aluminum) is an element which is contained as a deoxidizer formolten steel. However, when a content of Al exceeds 0.30%, ductility ata high temperature deteriorates. Accordingly, the Al content is set to0.30% or less. The Al content is preferably 0.25% or less, and morepreferably 0.20% or less. When it is desired to obtain the advantageouseffect, the Al content is preferably set to 0.01% or more, and morepreferably set to 0.02% or more.

N: less than 0.10%

N (nitrogen) is an element having an action of making the austeniticstructure stable, and is an element inevitably contained when anordinary melting method is adopted. However, in the present inventionwhere Ti is contained as an indispensable element, it is preferable toreduce a content of N as much as possible so as to prevent Ti from beingconsumed by the formation of TiN. However, in the case of atmosphericmelting, it is difficult to extremely reduce the N content. Accordingly,the N content is set to less than 0.10%.

In the chemical composition of the austenitic heat resistant alloy ofthe present invention, the balance consists of Fe and impurities. It ispreferable to set a content of Fe to 0.1 to 40.0%. In this embodiment,“impurity” means a component which is mixed in industrially producingthe alloy due to various causes, such as raw materials including ores orscrap, or production steps, and which is allowed to be mixed withoutadversely affecting the present invention.

The austenitic heat resistant alloy of the present invention may furthercontain one or more kinds selected from a group consisting of Mg, Ca,REM, V, B, Zr, Hf, Ta, and Re.

Any of Mg, Ca or REM has an action of fixing S as sulfides to enhancehigh temperature ductility. Accordingly, when it is desired to obtaingreater high temperature ductility, one or more kinds of these elementsmay be positively contained within the following range.

Mg: 0.05% or less

Mg (magnesium) has an action of fixing S, which inhibits ductility at ahigh temperature, as sulfides, thus improving high temperatureductility. Accordingly, Mg may be contained so as to obtain thisadvantageous effect. However, when a content of Mg exceeds 0.05%,cleanliness is lowered, and high temperature ductility is impaired onthe contrary. Accordingly, when Mg is contained, the amount of Mg is setto 0.05% or less. The Mg content is more preferably set to 0.02% orless, and further preferably set to 0.01% or less. On the other hand, toobtain the advantageous effect with certainty, the Mg content ispreferably set to 0.0005% or more, and more preferably set to 0.001% ormore.

Ca: 0.05% or less

Ca (calcium) has an action of fixing S, which inhibits ductility at ahigh temperature, as sulfides, thus improving high temperatureductility. Accordingly, Ca may be contained so as to obtain thisadvantageous effect. However, when a content of Ca exceeds 0.05%,cleanliness is lowered, and high temperature ductility is impaired onthe contrary. Accordingly, when Ca is contained, the amount of Ca is setto 0.05% or less. The Ca content is more preferably set to 0.02% orless, and further preferably set to 0.01% or less. On the other hand, toobtain the advantageous effect with certainty, the Ca content ispreferably set to 0.0005% or more, and more preferably set to 0.001% ormore.

REM: 0.50% or less

REM has an action of fixing S as sulfides, thus improving hightemperature ductility. REM also has an action of improving adhesivenessof a Cr₂O₃ protection film on a steel surface, thus improving oxidationresistance particularly when the alloy is repeatedly oxidized. Further,REM contributes to strengthening grain boundaries, thus having an actionof enhancing creep rupture strength and creep rupture ductility.However, when a content of REM exceeds 0.50%, the amount of inclusions,such as an oxide increases and hence, workability and weldability areimpaired. Accordingly, when REM is contained, the amount of REM is setto 0.50% or less. The REM content is more preferably set to 0.30% orless, and further preferably set to 0.15% or less. On the other hand, toobtain the advantageous effects with certainty, the REM content ispreferably set to 0.0005% or more, more preferably set to 0.001% ormore, and further preferably set to 0.002% or more.

REM indicates 17 elements in total, including Sc, Y, and thelanthanoids. The REM content means the total content of these elements.

The total content of Mg, Ca and REM may be 0.6% or less. However, thetotal content is more preferably 0.4% or less, and further preferably0.2% or less.

Any of V, B, Zr, or Hf has an action of enhancing high temperaturestrength and creep rupture strength. Accordingly, when it is desired toobtain greater high temperature strength and greater creep rupturestrength, the alloy may positively contain one or more kinds of theseelements within the following range.

V: 1.5% or less

V (vanadium) has an action of forming carbo-nitrides to enhance hightemperature strength and creep rupture strength. Accordingly, V may becontained so as to obtain these advantageous effects. However, when acontent of V exceeds 1.5%, high temperature corrosion resistance islowered and, further, ductility and toughness deteriorate due to theprecipitation of a brittle phase. Accordingly, when V is contained, theamount of V is set to 1.5% or less. The V content is more preferably setto 1.0% or less. On the other hand, to obtain the advantageous effectwith certainty, the V content is preferably set to 0.02% or more, andmore preferably set to 0.04% or more.

B: 0.01% or less

B (boron) is present in carbide or in a matrix. B has not only an actionof promoting micronization of precipitated carbide, but also an actionof strengthening grain boundaries, thus enhancing creep rupturestrength. However, when a content of B exceeds 0.01%, ductility at ahigh temperature is lowered, and a fusing point is also lowered.Accordingly, when B is contained, the amount of B is set to 0.01% orless. The B content is more preferably 0.008% or less, and furtherpreferably 0.006% or less. On the other hand, to obtain the advantageouseffects with certainty, the B content is preferably set to 0.0005% ormore, more preferably set to 0.001% or more, and further preferably setto 0.0015% or more.

Zr: 0.10% or less

Zr (zirconium) is an element which promotes micronization ofcarbo-nitrides, and which enhances creep rupture strength as a grainboundary strengthening element. However, when a content of Zr exceeds0.10%, ductility at a high temperature is lowered. Accordingly, when Zris contained, the amount of Zr is set to 0.10% or less. The Zr contentis more preferably 0.06% or less, and further preferably 0.05% or less.On the other hand, to obtain the advantageous effects with certainty,the Zr content is preferably set to 0.005% or more, and more preferablyset to 0.01% or more.

Hf: 1.0% or less

Hf (hafnium) has an action of contributing to strengtheningprecipitation as carbo-nitrides, thus enhancing creep rupture strength.Accordingly, Hf may be contained so as to obtain these advantageouseffects. However, when a content of Hf exceeds 1.0%, workability andweldability are impaired. Accordingly, when Hf is contained, the amountof Hf is set to 1.0% or less. The Hf content is more preferably set to0.8% or less, and further preferably set to 0.5% or less. On the otherhand, to obtain the advantageous effects with certainty, the Hf contentis preferably set to 0.005% or more, more preferably set to 0.01% ormore, and further preferably set to 0.02% or more.

The total content of V, B, Zr, and Hf is preferably 2.6% or less, andmore preferably 1.8% or less.

Either one of Ta or Re dissolves in austenite forming a matrix, thushaving an action of solid-solution strengthening. Accordingly, when itis desired to obtain greater high temperature strength and creep rupturestrength due to an action of solid-solution strengthening, one or bothof these elements may be positively contained within the followingrange.

Ta: 8.0% or less

Ta (tantalum) has an action of forming carbo-nitrides, and also has anaction of enhancing high temperature strength and creep rupture strengthas a solid-solution strengthening element. Accordingly, Ta may becontained so as to obtain these advantageous effects. However, when acontent of Ta exceeds 8.0%, workability and mechanical properties areimpaired. Accordingly, when Ta is contained, the amount of Ta is set to8.0% or less. The Ta content is more preferably set to 7.0% or less, andfurther preferably set to 6.0% or less. On the other hand, to obtain theadvantageous effects with certainty, the Ta content is preferably set to0.01% or more, more preferably set to 0.1% or more, and furtherpreferably set to 0.5% or more.

Re: 8.0% or less

Re (rhenium) has an action of enhancing high temperature strength andcreep rupture strength mainly as a solid-solution strengthening element.Accordingly, Re may be contained so as to obtain these advantageouseffects. However, when a content of Re exceeds 8.0%, workability andmechanical properties are impaired. Accordingly, when Re is contained,the amount of Re is set to 8.0% or less. The Re content is morepreferably set to 7.0% or less, and further preferably set to 6.0%. Onthe other hand, to obtain the advantageous effects with certainty, theRe content is preferably set to 0.01% or more, more preferably set to0.1% or more, and further preferably set to 0.5% or more.

The total content of Ta and Re is preferably 14.0% or less, and morepreferably 12.0% or less.

2. Grain Size

Austenite grain size number at outer surface portion: −2.0 to 4.0

When an austenitic grain size at an outer surface portion is extremelylarge, 0.2% proof stress and tensile strength at a normal temperatureare lowered. On the other hand, when an austenitic grain size at anouter surface portion is extremely small, it becomes impossible tomaintain high creep rupture strength at a high temperature. Accordingly,the austenite grain size number at the outer surface portion is set to avalue ranging from −2.0 to 4.0. In a production process for a Ni-basedalloy, by properly adjusting a heat-treatment temperature and holdingtime after hot working and a cooling method, it is possible to set thegrain size number at the outer surface portion to a value which fallswithin the range after final heat treatment.

3. Size Shortest Distance From Center Portion to Outer Surface Portion:40 mm or More

As described above, in a large-sized structural member, in addition to aproblem that 0.2% proof stress and tensile strength at a normaltemperature are lowered, there is also a problem that creep rupturestrength varies from region to region. However, the austenitic heatresistant alloy according to the present invention exhibits sufficient0.2% proof stress and tensile strength at a normal temperature, andsufficient creep rupture strength at a high temperature in large-sizedstructural members. That is, the present invention can obtain remarkableadvantageous effects in members having a thick wall.

Accordingly, in the austenitic heat resistant alloy of the presentinvention, the shortest distance from the center portion to the outersurface portion of a cross section is set to 40 mm or more, the crosssection being perpendicular to a longitudinal direction. To obtain moreremarkable advantageous effects of the present invention, the shortestdistance from the center portion to the outer surface portion ispreferably 80 mm or more, and more preferably 100 mm or more. In thisembodiment, the shortest distance from the center portion to the outersurface portion refers to a radius (mm) of a cross section when an alloyhas a columnar shape, and the shortest distance refers to a half-length(mm) of the short side of a cross section when an alloy has aquadrangular prism shape, for example.

As described later, the heat resistant alloy according to the presentinvention is obtained by performing hot working, such as hot forging orhot rolling on an ingot, or a cast piece, obtained by continuous castingor the like, for example. When an ingot is used, the longitudinaldirection of a heat resistant alloy substantially refers to a directionalong which a top portion and a bottom portion of the ingot areconnected. When a cast piece is used, the longitudinal direction of aheat resistant alloy substantially refers to the longitudinal directionof the cast piece.

4. Amount of Cr Which is Present as Precipitate Obtained by ExtractionResidue Analysis

Cr_(PB)/Cr_(PS)23 10.0   (i)

where meaning of each symbol in the formula (i) is as follows:

Cr_(PB): amount of Cr which is present at center portion as precipitateobtained by extraction residue analysis

Cr_(PS): amount of Cr which is present at outer surface portion asprecipitate obtained by extraction residue analysis

In a production process for an alloy, after heat treatment, which isperformed after the hot working, is performed, undissolved Crprecipitations (mainly carbides) are generated at crystal grainboundaries or within grains. Particularly at the center portion of thealloy, a cooling speed is slower than that at the outer surface portionof the alloy and hence, the amount of Cr precipitates tends to increase.Accordingly, when a value of Cr_(PB)/Cr_(PS) exceeds 10.0, it becomesimpossible to maintain high creep rupture strength at a hightemperature. On the other hand, it is not necessary to set the lowerlimit value of Cr_(PB)/Cr_(PS). However, there is a tendency that theamount of precipitates increases more at the center portion than at theouter surface portion and hence, Cr_(PB)/Cr_(PS) is preferably set to1.0 or more.

An extraction residue analysis is performed by the following procedure.First, test coupons for measuring Cr precipitates are obtained from thecenter portion and the outer surface portion of the cross section of analloy specimen, the cross section being perpendicular to thelongitudinal direction of the alloy specimen. The surface area of eachtest coupon is obtained and, thereafter, only the base metal of thealloy specimen is completely electrolyzed in a 10% acetylacetone—1%tetramethyl ammonium chloride—methanol solution under an electrolysiscondition of 20 mA/cm². Then, the solution after electrolysis isperformed is filtered through a 0.2 μm filter to extract precipitates asa residue. Thereafter, the extracted residue is decomposed with an acid,and is analyzed using an inductively coupled plasma emissionspectrophotometer (ICP-AES) to measure a content (mass %) of Crcontained as undissolved Cr precipitate, and a value of Cr_(PB)/Cr_(PS)is obtained based on the measured value.

5. Mechanical Properties

YS_(S)/YS_(B)≤1.5   (ii)

TS_(S)/TS_(B)≤1.2   (iii)

where meaning of each symbol in the formulas is as follows:

YS_(B): 0.2% proof stress at center portion

YS_(S): 0.2% proof stress at outer surface portion

TS_(B): tensile strength at center portion

TS_(S): tensile strength at outer surface portion

In a large-sized structural member, a cooling speed at the time ofperforming heat treatment varies from region to region and hence, thereis a tendency that great variations occur in mechanical properties fromregion to region due to the difference in the cooling speed. If there isa large difference in 0.2% proof stress and tensile strength at a normaltemperature between the center portion and the outer surface portion ofthe large-sized structural member, there arises a problem that someregions do not satisfy the specifications.

Accordingly, with respect to the austenitic heat resistant alloyaccording to the present invention, mechanical properties at a normaltemperature satisfy the formula (ii) and formula (iii). It is notnecessary to set the respective lower limit values of these formulas.However, there is a tendency that mechanical characteristics at thecenter portion are inferior to mechanical characteristics at the outersurface portion and hence, either one of formula (ii) or formula (iii)is preferably set to 1.0 or more.

0.2% proof stress and tensile strength are obtained in such a way thatround bar tensile test coupons, each having a parallel portion with alength of 40 mm, are cut out by mechanical processing from the centerportion and the outer surface portion of the alloy parallel to thelongitudinal direction, and a tensile test is performed on these testcoupons at a room temperature. The tensile test is performed inaccordance with JIS Z 2241 (2011).

6. Creep Rupture Strength

The austenitic heat resistant alloy of the present invention is used ina high temperature environment, thus being required to be excellent inhigh temperature strength, particularly, in creep rupture strength.Accordingly, 10,000-hour creep rupture strength at 700° C. in thelongitudinal direction is preferably 100 MPa or more at the centerportion of the heat resistant alloy of the present invention.

Creep rupture strength is obtained by the following method. First, roundbar creep rupture test coupons, described in JIS Z 2241 (2011), andhaving a diameter of 6 mm and a gage length of 30 mm, are cut out bymechanical processing from the center portions of the alloys parallel tothe longitudinal direction. Then, a creep rupture test is performed inthe atmosphere of 700° C., 750° C., and 800° C. to obtain 10,000-hourcreep rupture strength at 700° C. by a Larson-Miller parameter method.The creep rupture test is performed in accordance with JIS Z 2271(2010).

7. Production Method

The austenitic heat resistant alloy of the present invention can beproduced by performing hot working on an ingot or a cast piece havingthe above chemical composition. In the above step of performing hotworking, processing is performed such that the longitudinal direction ofthe alloy in the final shape aligns with the longitudinal direction ofthe ingot or the cast piece forming a starting material. Hot working maybe performed only in the longitudinal direction. However, to obtain amore uniform micro-structure at a higher working ratio, hot working maybe performed one or more times in a direction substantiallyperpendicular to the longitudinal direction. After the hot working isperformed, hot working of another method, such as hot extrusion, may befurther performed when necessary.

In producing the austenitic heat resistant alloy of the presentinvention, after the above step, final heat treatment described below isperformed so as to minimize variation in metal micro-structure andmechanical properties from region to region, thus maintaining high creeprupture strength.

First, the alloy on which hot working was performed is heated to aheat-treatment temperature T (° C.) ranging from 1100 to 1250° C., andis held for 1000 D/T to 1400 D/T (min) within such a range. In thisembodiment, symbol “D” denotes the diameter (mm) of the alloy when thealloy has a columnar shape, and “D” denotes a diagonal distance (mm)when the alloy has a quadrangular prism shape, for example. That is,symbol “D” denotes the maximum value (mm) of a linear distance betweenan arbitrary point on the outer edge of the cross section of the alloyand another arbitrary point on the outer edge, the cross section beingperpendicular to a longitudinal direction of the alloy.

When the heat-treatment temperature is less than 1100° C., the amount ofundissolved chromium carbide or the like increases, thus lowering creeprupture strength. On the other hand, when the heat-treatment temperatureexceeds 1250° C., grain boundaries are dissolved or grains areremarkably coarsened so that ductility is lowered. Accordingly, it ismore desirable to set the heat-treatment temperature to 1150° C. orabove and 1230° C. or below. Further, when the holding time is less than1000 D/T (min), undissolved chromium carbide at the center portionincreases so that Cr_(PB)/Cr_(PS) falls outside a range defined by thepresent invention. On the other hand, when the holding time exceeds 1400D/T (min), grain at the outer surface portion is coarsened so that theaustenite grain size number falls outside the range defined by thepresent invention.

Immediately after the alloy is heated and held, the alloy is cooled withwater. This is because when a cooling speed becomes lower, particularlyat the center portion of the alloy, a large amount of undissolved Crprecipitates is generated at crystal grain boundaries or within grainsso that there is a possibility that the formula (i) is not satisfied.

Hereinafter, the present invention is described more specifically withreference to examples. However, the present invention is not limited tothese examples.

EXAMPLE

Alloys having the chemical compositions shown in Table 1 were melted ina high-frequency vacuum furnace to prepare ingots each having an outerdiameter of 550 mm, and a weight of 3t.

TABLE 1 Chemical composition (in mass %, balance: Fe and impurities)Alloy C Si Mn P S Cr Ni Co W Ti Nb 1 0.075 0.38 1.12 0.008 0.001 21.541.3 — 4.6 0.41 0.73 2 0.043 0.42 0.95 0.006 0.002 25.3 44.2 7.3 8.40.22 0.45 3 0.090 0.40 1.07 0.010 0.001 26.8 48.5 — 6.1 0.15 0.29 40.041 0.43 1.24 0.009 0.003 24.6 51.1 — 5.2 0.17 0.24 5 0.030 0.51 1.060.011 0.002 27.5 53.7 — 4.8 0.25 0.71 6 0.064 0.24 1.57 0.014 0.001 23.447.2 — 6.4 0.47 0.60 7 0.102 0.78 0.59 0.008 0.001 25.6 50.6 — 5.7 0.190.43 8 0.056 0.43 1.25 0.012 0.001 20.9 38.4 — 4.9 0.20 0.39 9 0.0480.69 1.68 0.015 0.002 24.7 52.1 — 6.5 0.34 0.25 A 0.073 0.40 1.08 0.0070.001 21.7 41.6 — 4.5 0.45 0.73 B 0.076 0.42 1.10 0.008 0.001 21.4 41.0— 4.8 0.41 0.75 C 0.045 0.39 1.05 0.008 0.002 25.0 45.2 7.0 8.2 0.230.41 D 0.044 0.40 1.01 0.007 0.002 24.9 44.8 7.1 8.0 0.24 0.42 E 0.0440.42 0.98 0.007 0.001 25.2 45.7 7.4 8.1 0.24 0.44 Chemical composition(in mass %, balance: Fe and impurities) Alloy Mo Cu Al N Mg Ca REM V BOthers 1 0.08 0.15 0.14 0.031 — — — — — — 2 0.05 0.07 0.03 0.015 — — — —0.0051 — 3 0.06 0.11 0.25 0.026 0.0012 0.002 0.01 0.6 — Zr: 0.01, Ta:1.4 4 0.13 0.08 0.09 0.019 — — 0.06 — 0.0063 Hf: 0.3, Re: 1.2 5 0.340.21 0.16 0.024 — — 0.11 — — — 6 0.07 0.13 0.20 0.018 — — — — 0.0017 — 70.09 0.10 0.09 0.034 — — — — — Zr: 0.05 8 0.11 0.15 0.12 0.072 — — — 0.7— — 9 0.14 0.08 0.17 0.044 — 0.009 — — — — A 0.05 0.14 0.10 0.035 — — —— — — B 0.07 0.15 0.11 0.040 — — — — — — C 0.08 0.07 0.05 0.025 — — — —0.0050 — D 0.08 0.08 0.05 0.019 — — — — 0.0052 — E 0.07 0.08 0.04 0.018— — — — 0.0053 —

The obtained ingots were processed to have a columnar shape with anouter diameter of 120 to 480 mm by hot forging, and final heat treatmentwas performed under conditions shown in Table 2 to obtain alloy memberspecimens. Alloys 1, 2 and 4 were subjected to forging in a directionsubstantially perpendicular to the longitudinal direction after hotforging in the longitudinal direction and before final heat treatmentand, thereafter, final hot forging was further performed in thelongitudinal direction.

TABLE 2 Outer Heat-treatment Holding diameter D temperature T 1000 1400time Cooling Alloy (mm) (° C.) D/T D/T (min) method 1 450 1180 381 534480 water cooling 2 350 1200 292 408 360 water cooling 3 200 1150 174243 220 water cooling 4 480 1150 417 584 540 water cooling 5 250 1210207 289 260 water cooling 6 300 1200 250 350 310 water cooling 7 1201180 102 142 130 water cooling 8 300 1180 254 356 295 water cooling 9520 1200 433 607 570 water cooling A 450 1180 381 534   660 ** watercooling B 450 1180 381 534   200 ** water cooling C 350   1070 ** 327458 340 water cooling D 350   1270 ** 276 386 340 water cooling E 3501200 292 408 360 air cooling ** ** indicates that production conditionsdo not satisfy those defined by the present invention.

A test coupon for observing micro-structure was obtained from the outersurface portion of each specimen, and the cross section in thelongitudinal direction was polished with emery paper and a buff.Thereafter, the test coupon was etched with a mixed acid, and opticalmicroscopic observation was performed. The grain size number on anobservation surface was obtained in accordance with a determinationmethod defined by JIS G 0551 (2013) where the grain size number isdetermined based on crossing line segments (grain size).

Next, test coupons for measuring the amount of Cr precipitates wereobtained from the center portion and the outer surface portion of thecross section of each specimen, the cross section being perpendicular tothe longitudinal direction of the specimen. The surface area of eachtest coupon was obtained and, thereafter, only the base metal of thealloy specimen was completely electrolyzed in a 10% acetylacetone—1%tetramethyl ammonium chloride—methanol solution under an electrolysiscondition of 20 mA/cm². Then, the solution after electrolysis wasperformed was filtered through a 0.2 μm filter to extract precipitatesas a residue. Thereafter, extracted residue was decomposed with an acid,and was subjected to ICP-AES measurement to measure a content (mass %)of Cr contained as undissolved Cr precipitate and, then, a value ofCr_(PB)/Cr_(PS) was obtained based on the measured value.

Tensile test coupons, each having a parallel portion with a length of 40mm, were cut out by mechanical processing from the center portion andthe outer surface portion of each specimen parallel to the longitudinaldirection, and a tensile test was performed on these test coupons at aroom temperature so as to obtain 0.2% proof stress and tensile strength.Further, creep rupture test coupon, having a parallel portion with alength of 30 mm, was cut out by mechanical processing from the centerportion of each specimen parallel to the longitudinal direction. Then, acreep rupture test was performed in the atmosphere of 700° C., 750° C.,and 800° C. to obtain 10,000-hour creep rupture strength at 700° C. by aLarson-Miller parameter method.

These results are collectively shown in Table 3.

TABLE 3 Grain size number at Creep outer surface rupture Alloy portionCr_(PB)/Cr_(PS) YS_(S)/YS_(B) TS_(S)/TS_(B) strength^(#) 1 −1.1  6.9 1.21.0 112 Inventive 2 0.2 3.4 1.3 1.1 128 example 3 2.2 5.8 1.0 1.0 115 40.6 7.9 1.2 1.0 118 5 −0.4  6.5 1.2 1.1 116 6 −0.7  5.7 1.3 1.1 119 71.2 2.8 1.1 1.0 118 8 1.0 4.4 1.2 1.0 114 9 −1.3  8.7 1.4 1.2 118 A −2.5 * 6.0  1.6 *  1.3 * 110 Comparative B 3.5 4.6 1.1 1.1 92 example C 5.7 *  12.6 * 1.2 1.1 93 D  −2.8 * 2.4  1.6 *  1.4 * 97 E 0.5  14.8 *1.3 1.0 95 * indicates that conditions fall outside the range of thepresent invention. ^(#)indicates 10,000-hour creep rupture strengths at700° C.

The alloy A and the alloy B have substantially the same chemicalcomposition as the alloy 1, and are formed into a final shape same asthat of the alloy 1 by hot forging. However, a holding time in heattreatment falls outside the production conditions defined by the presentinvention. Due to such holding time, the alloy A has the result that thegrain size number at the outer surface portion falls outside the rangedefined by the present invention, and a value of YS_(S)/YS_(B) and avalue of TS_(S)/TS_(B) fall outside the range defined by the presentinvention. Accordingly, the alloy A has a large variation in mechanicalcharacteristics from region to region. The alloy B falls outside therange defined by the present invention with respect to creep rupturestrength and, as a result, creep rupture strength of the alloy B isremarkably lower than that of the alloy 1.

Alloys C, D, and E have substantially the same chemical composition asthe alloy 2, and are formed into a final shape same as that of the alloy2 by hot forging. The alloy C is lower than the range defined by thepresent invention with respect to the heat-treatment temperature andhence, the grain size number at the outer surface portion and a value ofCr_(PB)/Cr_(PS) fall outside the ranges defined by the presentinvention. As a result, creep rupture strength of the alloy C isremarkably lower than that of the alloy 2.

The alloy D is higher than the range defined by the present inventionwith respect to a heat-treatment temperature and hence, the grain sizenumber at the outer surface portion and a value of YS_(S)/YS_(B) and avalue of TS_(S)/TS_(B) fall outside the range defined by the presentinvention. As a result, creep rupture strength of the alloy D isremarkably lower than that of the alloy 2.

With regard to the alloy E, a cooling method in final heat treatment wasnot water cooling but was air cooling and hence, a cooling speed wasremarkably low. Accordingly a value of Cr_(PB)/Cr_(PS) falls outside therange defined by the present invention and, as a result, creep rupturestrength of the alloy E is remarkably lower than that of the alloy 2. Onthe other hand, the alloys 1 to 9 which satisfy all specifications ofthe present invention have small variation in mechanicalcharacteristics, and favorable creep rupture strength.

INDUSTRIAL APPLICABILITY

The austenitic heat resistant alloy of the present invention has smallvariation in mechanical properties from region to region, and isexcellent in creep rupture strength at a high temperature. Accordingly,the austenitic heat resistant alloy of the present invention ispreferably applicable to a large-sized structural member for a thermalpower generation boiler, a chemical plant or the like which is used in ahigh temperature environment.

1. An austenitic heat resistant alloy having a chemical compositionconsisting of, in mass %: C: 0.02 to 0.12%; Si: 2.0% or less; Mn: 3.0%or less; P: 0.030% or less; S: 0.015% or less; Cr: 20.0% or more andless than 28.0%; Ni: more than 35.0% and 55.0% or less; Co: 0 to 20.0%;W: 4.0 to 10.0%; Ti: 0.01 to 0.50%; Nb: 0.01 to 1.0%; Mo: less than0.50%; Cu: less than 0.50%; Al: 0.30% or less; N: less than 0.10%; Mg: 0to 0.05%; Ca: 0 to 0.05%; REM: 0 to 0.50%; V: 0 to 1.5%; B: 0 to 0.01%;Zr: 0 to 0.10%; Hf: 0 to 1.0%; Ta: 0 to 8.0%; Re: 0 to 8.0%; and thebalance: Fe and impurities, wherein a shortest distance from a centerportion to an outer surface portion of a cross section of the alloy is40 mm or more, the cross section being perpendicular to a longitudinaldirection of the alloy, an austenite grain size number at the outersurface portion is −2.0 to 4.0, an amount of Cr which is present as aprecipitate obtained by an extraction residue analysis satisfies afollowing formula (i), and mechanical properties at a normal temperaturesatisfy following formula (ii) and formulaCr_(PB)/Cr_(PS)≤10.0   (i)YS_(S)/YS_(B)≤1.5   (ii)TS_(S)/TS_(B)≤1.2   (iii) where meaning of each symbol in the formulasis as follows: Cr_(PB): amount of Cr which is present at center portionas precipitate obtained by extraction residue analysis Cr_(PS): amountof Cr which is present at outer surface portion as precipitate obtainedby extraction residue analysis YS_(B): 0.2% proof stress at centerportion YS_(S): 0.2% proof stress at outer surface portion TS_(B):tensile strength at center portion TS_(S): tensile strength at outersurface portion.
 2. The austenitic heat resistant alloy according toclaim 1, wherein the chemical composition contains one or more elementsselected from a group consisting of, in mass %: Mg: 0.0005 to 0.05%; Ca:0.0005 to 0.05%; REM: 0.0005 to 0.50%; V: 0.02 to 1.5%; B: 0.0005 to0.01%; Zr: 0.005 to 0.10%; Hf: 0.005 to 1.0%; Ta: 0.01 to 8.0%; and Re:0.01 to 8.0%.
 3. The austenitic heat resistant alloy according to claim1, wherein 10,000-hour creep rupture strength at 700° C. in thelongitudinal direction at the center portion is 100 MPa or more.
 4. Amethod for producing an austenitic heat resistant alloy, the methodcomprising the steps of: performing hot working on an ingot or a castpiece having the chemical composition according to claim 1; andthereafter performing heat treatment where the ingot or the cast pieceis heated to a heat-treatment temperature T (° C.) ranging from 1100 to1250° C., is held for 1000 D/T to 1400 D/T (min), and is cooled withwater, wherein symbol “D” denotes a maximum value (mm) of a lineardistance between an arbitrary point on an outer edge of a cross sectionof the alloy and another arbitrary point on the outer edge, the crosssection being perpendicular to a longitudinal direction of the alloy. 5.The method for producing an austenitic heat resistant alloy according toclaim 4, wherein in the step of performing the hot working, the workingis performed one or more times in a direction substantiallyperpendicular to the longitudinal direction.
 6. The austenitic heatresistant alloy according to claim 2, wherein 10,000-hour creep rupturestrength at 700° C. in the longitudinal direction at the center portionis 100 MPa or more.
 7. A method for producing an austenitic heatresistant alloy, the method comprising the steps of: performing hotworking on an ingot or a cast piece having the chemical compositionaccording to claim 2; and thereafter performing heat treatment where theingot or the cast piece is heated to a heat-treatment temperature T (°C.) ranging from 1100 to 1250° C., is held for 1000 D/T to 1400 D/T(min), and is cooled with water, wherein symbol “D” denotes a maximumvalue (mm) of a linear distance between an arbitrary point on an outeredge of a cross section of the alloy and another arbitrary point on theouter edge, the cross section being perpendicular to a longitudinaldirection of the alloy.
 8. The method for producing an austenitic heatresistant alloy according to claim 7, wherein in the step of performingthe hot working, the working is performed one or more times in adirection substantially perpendicular to the longitudinal direction.